As a method for inspecting a surface shape of an object, there is a method for evaluating a surface shape of an object to be inspected, by projecting a stripe pattern having periodical bright and dark lines on the object to be inspected and measuring deviation of the bright-dark period in a reflection image produced by reflection at a front surface of the object to be inspected (for example, refer to Patent Document 1). However, when such a method is applied to a transparent plate-shaped object such as a glass plate, not only a reflection image produced by a front surface of the transparent plate-shaped object but also a reflection image produced by a rear surface of the transparent plate-shaped object are captured at the same time. From now, a reflection image from a front surface of a transparent plate-shaped object is designated as a front surface reflection image, and a reflection image from a rear surface of a transparent plate-shaped object is designated as a rear surface reflection image.
FIG. 16 is an explanation view showing that a front surface reflection image and a rear surface reflection image are formed at the same time. As shown in FIG. 16, light emitted from a point 5 on a stripe pattern is reflected by a front surface 3a of a transparent plate-shaped object 3, and focused at a capturing point 10 on a photo-receiving plane 7 of a camera through an optical path 8. Further, light transmitted through the transparent plate-shaped object 3 is reflected by a rear surface 3b of the transparent plate-shaped object 3 and focused at a capturing point 11 on the photo-receiving plane 7 through an optical path 9.
Here, depending on a period or width of the stripe pattern, there is a case that the following problems occur to a captured image signal. FIG. 17 is a wave pattern view showing an example of image signal output from a camera. FIG. 17(a) shows an image signal of front surface reflection image, and FIG. 17(b) shows an image signal of rear surface reflection image.
Further, low level indicates a level of image signal of a dark portion of the stripe pattern, and high level indicates a level of image signal of a bright portion of the stripe pattern. If the width of a dark portion of the stripe pattern is wide, the width of a low level portion in the image signal becomes wide and there is a case that a low level portion of an image signal of front surface reflection image overlaps with a low level portion of an image signal of rear surface reflection image. In this case, an image signal output from the camera becomes a signal as shown in FIG. 17(c), and inspection of front surface shape is carried out based on the signal different from the image signal (FIG. 17(a)) of front surface reflection image that is essentially required.
Further, as shown in FIG. 18, even if the width of dark portions of the stripe pattern is sufficiently narrow, if the distance T between the positions of dark portions of an image signal of front reflection image and the positions of dark portions of an image signal of rear surface reflection image, is close to an integer times of a bright-dark period of the stripe pattern, an image signal as shown in FIG. 18(c) is output from the camera. Here, FIG. 18(a) shows an image signal of front surface reflection image, and FIG. 18(b) shows an image signal of rear surface reflection image. Also in this case, inspection of surface shape is carried out based on a signal different from an image signal of front surface reflection image, which causes a problem that the surface shape cannot be inspected accurately.
As a method for solving the above problem, there are methods of weakening or removing the rear surface reflection image. For example, a method of using ultraviolet rays is one of these methods (refer to Non-Patent Document 1). Since the ultraviolet rays are absorbed in a glass, contrast of a rear surface reflection image becomes sufficiently lower than the contrast of front surface reflection image, and even if the rear reflection image overlaps with the front surface reflection image, it is possible to reduce the influence of rear surface reflection image.
Patent Document 1: JP-A-11-148813 (paragraphs 0082 to 0083 and FIG. 24)
Non-Patent Document 1: “Hikari Gijutsu Contact” (Optical and Electrooptical Engineering Contact) (published by Japan Optomechatronics Association, Vol. 39, No. 2 (year 2001), pages 103 to 110)
However, in order to realize a method of using ultraviolet rays, it is necessary to use a UV light source and to constitute a camera by special materials transmitting ultraviolet rays, which increases the cost of inspection apparatus. Further, not all wavelength of ultraviolet rays is usable for the method but it is necessary to select a wavelength region that is sufficiently absorbable into a glass in order to weaken a rear surface reflection image. There is a type of glass having a specific composition which absorbs little ultraviolet rays, for which this method can not be applied.
Under these circumstances, it is an object of the present invention to provide a method and an apparatus for inspecting front surface shape, which has an inexpensive construction, which can remove the influence of rear surface reflection image, and which can accurately inspect characteristics of front surface shape.